(b) Authority of non-resident defendant's attorney. The attorney for a non-resident defendant in an action affecting real property shall file with the clerk written authority for his appearance, executed and acknowledged in the form required to entitle a deed to be recorded, and shall serve either a copy of such authority or notice of such filing on the plaintiff's attorney within twenty days after appearing or making a motion.
(c) Agencies or wholly-owned corporations of the United States. This rule does not apply to an attorney representing an official, agency or instrumentality of, or corporation wholly owned by, the United States.